Cesium evaporator

ABSTRACT

Cesium evaporator filled with a mixed powder of a silicon and cesium chromate and/or cesium bichromate for depositing cesium vapor on the face plate of a photoelectric tube, the silicon powder has such a particle size distribution that the average particle size is 30 to 80 microns, with particles larger than 100 microns or those smaller than one micron constituting not more than 30 weight percent, and the cesium chromate or cesium bichromate powder has such a particle size distribution that the average particle size is 3 to 10 microns with particles larger than 25 microns constituting not more than 30 weight percent and those smaller than 0.5 micron constituting not more than 40 weight percent.

United States Patent [151 3,644,101 Takashio et al. Feb. 22, 1972 [54]CESIUM EVAPORATOR 1,835,118 12/1931 Marden et al ....75/66 1,966,2207/1934 Rentschler ....75/66 [721 Invenmrs- EF Takaymh 1,966,254 7/1934Marden et al..... ....75/66 bmh Japan 3,468,807 9/1969 Spangenberg....75/66 [73] Assignee; Tokyo 1 Shibaum m Co Ltd 3,468,806 9/1969Nlewold ..75/66 Kawasaki-shi, Japan Primary Examiner-Norman YudkoffFlledi June 1968 Assistant Examiner-S. Silverberg [211 App. No; 734,963AttameyGeorge B. Ou evolk [57] ABSTRACT [30] Fomgn Apphcatlon Pnomy Data1 Cesium evaporator filled with a mixed powder of a silicon and June 10,1967 Japan ..42/3677 cesium 'chromate and/or cesium bichromate fordepositing I cesium vapor on the face plate of a photoelectric tube, thesil- 52 U.S.Cl ..23/294,117l225,75/66 icon'powder has Such a ParticleSize distribution that the [51] Int. Cl. R ..B44d l/02,C23d 1/02 averageParticle Size is 30 to 80 microns with ParticleS larger [58] w of Search117/107, 106, 34 1072, 223 than 100 microns or those smaller than onemicron constitut- 117/224, 75 m 23/294; 350/257; ing not more than 30weight percent, and the cesium chro- 252/1883 mate or cesium bichromatepowder has such a particle size distribution that the average particlesize is 3 to microns R ed with particles largerthan microns constitutingnot more [56] defences than weight percent and those smaller than 0.5micron con- UNITED STATES PATENTS stituting not more than weightpercent. 1,224,339 5/1917 1 Ciaims, 8 Drawing Figures Darrah ..ll7/l07PATENTEDFEB 22 I972 SHEET 1 BF 3 FIG.I

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INVENTORS BY W PATENTE [IFEBZZ I972 SHEET 3 [IF 3 2I sI LIc0N POWDERCESIUM CHROMATE 22 I 0R CESIUM BI- J CHROMATE POWDER WASHING WITH ACID IWASHING PULVERIZING WITH WATER AND DRYING I 25- DRYING I 26 PULVERIZINGS1 S|EVING P331 AND DRYING I 27\VACUUM TREATMENT I I 28* sIEvING 2ndsIEvING. -32 I I I MIXING -33 I DRYING -34 I S-IEVING -35 I MIXING -36MIXTURE P37 I H1 TM M INVENTORS CULT/WWW CESIUM EVAPORATOR The presentinvention relates to a cesium evaporator for depositing cesium, vapor onthe face plate of a photoelectric tube, and more particularly to acesium evaporator capable of evaporating cesium at a constant rate.

In depositing cesium vapor on the face plate of a photoelectric tube, itis generally the practice to use an evaporator having a lead wire fittedto each end and filled with a mixed powder of an appropriate reducingagent and cesium chromate and/or cesium bichromate. Disposed at asuitable distance from the face plate of the photoelectric tube, thecesium evaporator then is electrically heated through the lead wires toproduce the vapor of reduced cesium. The vapor of cesium thus producedis emitted through the holes provided on the sidewall of the evaporatorand is deposited on a thin layer, for example, of antimony orsilver-bismuth already coated on the face plate of the photoelectrictube. It is well known that face plate of high photosensitivity can beobtained only when the face plate is such that the face plate does notdisplay its photosensitivity until a layer of cesium is deposited on theface plate preliminary coated with a metal base such as antimony orsilver-bismuth.

To prepare in good yield a face plate of uniform high photosensitivity,it is required that the volume of cesium vapor generated by theevaporator, composition of the reactants present therein and velocity ofcesium deposition on the metal base per unit time are perfectly constantuntil completion of said deposition. However, in the prior art, thecesium evaporator is filled indiscriminately with mixed powders ofcesium chromate and/or cesium bichromate and at least one elementselected from the group consisting of such reducing agents as zirconium,iron, calcium, tungsten, silicon, aluminum and titanium, and sufficientattention is not given to particle size distribution of these powders,absorption of water and/or gas thereto, and surface conditions of thepowder particles. For example, the average particle size of the powdersto be mixed is not fully regulated, and there is included a large amountof relatively smaller or larger particles. This leads to varying contactareas among the powder particles filled into the evaporator, causing anuneven reaction velocity. Further, deposition of water on the particlesin turn causes their mutual adsorption or agglomeration, thus resultingin nonuniform fluidity and diffusion velocity of powders. Also theadsorption of gases or presence of oxides causes the evaporation ofreaction products other than cesium and hinders the production of cesiumvapor. As a result, it has been impossible to regulate the reactionvelocity and the evaporator itself sometimes tends to be damaged by anabrupt reaction. Therefore the prior art evaporator was handicapped byvarious drawbacks as listed above. Use of such cesium evaporatorpresents difficulties in controlling the reaction velocity, thusaffecting the properties of the face plate and of the secondaryelectron-multiplying electrode which the resultant decline in the yieldrate of a photoelectric tube.

The present invention has been accomplished with the view of providing auniform deposition of cesium vapor on the face plate of a photoelectrictube. The invention provides a cesium evaporator packed with a mixtureof silicon powder 30 to 80 microns in the average particle size andsubstantially free from water and adsorbed gases, the particle sizedistribution thereof being such that larger particles than 100 micronsand those smaller than 1 micron respectively account for not more than30 weight percent, and powder of cesium chromate and/or cesiumbichromate 3 to microns in the average particle size and substantiallyfree from water, the particle size distribution thereof being such thatlarger particles than 25 microns represent not more than 30 weightpercent, and those smaller than 0.5 micron constitute not more than 40weight percent.

The present invention can be more fully understood from the followingdetailed description taken in connection with the accompanying drawingsin which:

FIG. I is a perspective view of a cesium evaporator in common use;

FIG. 2 is a longitudinal section on line lI-II of the cesium evaporatorshown in FIG. 1;

FIGS. 3 and 4 are curve diagrams showing the relations between theparticle size distribution of cesium chromate powder and the yield rateof good quality face plates;

FIGS. 5 to 7 are curve diagrams showing the relations between theparticle size distribution of silicon powder and the yield rate of goodquality face plates; and

FIG. 8 is a block diagram of the process of obtaining a mixture ofdesired powders of silicon, cesium chromate and/or cesium bichromateaccording to an embodiment of the present invention.

The present inventors have found that when the particle sizedistributions of silicon powder, cesium chromate and/or cesiumbichromate are properly adjusted, and freed substantially of adsorbedwater with careful attention given to improving the surface condition ofthe silicon particles and jointly introduced into an evaporator, thefluidity and contact condition of powders and the diffusion velocitythereof at the time of reaction will become more constant, assuring agreater uniformity in the velocity of cesium vapor deposition on theface plate than has been possible with the prior art method.

There will now be described the process of the present invention byreference to the appended drawings. As shown in FIGS. 1 and 2, anevaporator 11 consists of a cylindrical body 12 made of a thin metalsheet and filled with a mixture 13 of powder of cesium chromate and/orcesium bichromate and that of silicon, both ends of the cylindrical bodybeing pressure-sealed, and fitted with a pair of lead wires 14, 14. Theoverlapped portion of the thin metal sheet 12 is joined together byequally spaced welds 15 forming between them a plurality of holes 16 toallow the cesium vapor produced in the evaporator 11 easily to scattertherefrom.

The present invention utilizes an evaporator of the same construction asdescribed above. However, the powder of cesium chromate or cesiumbichromate charged into the evaporator 11 has such a particle sizedistribution that the average particle size ranges between 3 and 10microns and that larger particles than 25 microns constitute not morethan 30 weight percent and those smaller than 0.5 micron account for notmore than 40 weight percent. If the powder of cesium chromate or cesiumbichromate has a smaller average particle size than 3 microns it tendsto agglomerate in itself with the resultant poor dispersion in thesilicon powder with which it is to be mixed. This will generally causethe cesium compounds to react with silicon rapidly and irregularly. Onthe other hand, where the particle size of powdered cesium compoundsexceeds 10 microns, reaction with silicon will sharply rise when thereaction proceeds to a certain interim stage, though substantially slowin the initial period, due to the smaller surface area of theseparticles. In the worst case the evaporator will be damaged by an abruptreaction.

Further, even though the average particle size may fall within the rangeof from 3 to 10 microns, if the powder of cesium compounds contains morethan 30 weight percent of particles smaller than 0.5 micron, thereaction velocity will not be constant due to the absence of uniformcontacts between the cesium compounds and silicon. Therefore, as shownin both FIGS. 3 and 4, the yield rate of good quality face plates(namely, the ratio of good quality product to the total output) willsharply decline. In FIG. 3 the ordinate represents the yield rate ofgood quality face plates and the abscissa the weight percent of smallerparticles than 0.5 micron included in the cesium chromate powder of 3 to10 microns in the average particle size. In FIG. 4, the ordinate denotesthe yield rate of good quality face plates and the abscissa the weightpercent of larger particles than 25 microns included in the cesiumchromate powder 3 to 10 microns in the average particle size. Thesetendencies shown in FIGS. 3 and 4 are also true with the cesiumbichromate powder. The term good quality product," as used in thisspecification, is defined to mean the face plate wherein the amount ofcesium vapor deposited in the aforementioned manner on the face plate byintroducing a prescribed quantity of electric current through theevaporator falls in the range of 30 to 50 percent of thestoichiometrical value to be produced by the reaction of cesium chromateor cesium bichromate with silicon. Accordingly, the 100 percentproduction of good quality face plates represents that the cesium vaporis deposited on the face plate at a perfectly uniform velocity. Theparticle sizes appearing throughout this application have been measuredby means of micromerograph.

The silicon powder used in the process of the present invention has sucha particle size distribution that the average particle size rangesbetween 30 and 80 microns and that those larger than 100 microns orthose smaller than 1 micron respectively account for not more than 30weight percent. The silicon powder of less than 30 microns in theaverage particle size is undesirable in that the surface of theparticles is vulnerable to oxidation. Conversely, where the averageparticle size exceeds 80 microns, the surface area of each particle willbe reduced relative to the volume thereof. Therefore, when the powder ofcesium chromate or cesium bichromate having a far smaller averageparticle size is mixed with such large silicon particles, there will notbe obtained a satisfactory contact between these two kinds of powders.For example, where, as shown in FIG. 5, the average particle size ofsilicon powder falls outside of the range of from 30 to 80 microns, theyield rate of good quality face plates is significantly reduced. In FIG.5, the ordinate represents the yield rate of good quality face platesand the abscissa the average particle size (micron) of silicon powder.If the silicon powder contains more than 30 weight percent of particleslarger than I microns and smaller than 1 micron respectively, eventhough the average particle size may fall within the range of from 30 to80 microns, these silicon particles will not have a uniform contact withthose of cesium chromate or cesium bichromate. The employment of suchsilicon powder leads to an uneven reaction velocity of cesium evaporatorand sharply reduces, as shown in FIGS. 6 and 7, the yield rate of goodquality face plates. In FIG. 6 the ordinate denotes the yield rate ofgood quality face plates and the abscissa the weight percent of largerparticles than 100 microns included in the silicon powder 30 to 80microns in the average particle size. In FIG. 7, the ordinate representsthe yield rate of good quality face plates and the abscissa the weightpercent of smaller particles than 1 micron contained in the siliconpowder 30 to 80 microns in the average particle size.

Not less important for the process of the present invention is thesubstantial elimination of adsorbed water, in addition to theaforementioned control of particle size distribution of powders ofcesium compounds and silicon. Particularly with respect to the siliconpowder, it is preferable to reduce adsorbed gases as much as possible byvacuum heating after pulverizing and drying. Regarding the powder ofcesium chromate or cesium bichromate, it is also advisable to repeatseveral times the cycle of pulverizing and drying so as to obtainwater-free powder having prescribed average particle size and particlesize distribution and thereafter sieve them using two screens ofdifferent mesh sizes to prevent the agglomeration of very fineparticles. Specifically, this sieving is preferably carried out firstwith a coarse screen, for example, of 60 Tyler mesh, and then with afiner screen, for example, of 200 Tyler mesh. If a fine screen is usedfrom the start, then even agglomerated minute particles will be presseddown and forced out through the mesh. Therefore, the aforementionedtwo-step sieving with a coarse and a fine screen is effective to preventsuch occurrence. To prevent the adsorption of moisture, the entire cycleof sieving is preferably carried out under the irradiation of anincandescent lamp or infrared-ray lamp.

The silicon powders processed to have a prescribed particle sizedistribution and surface condition by washing with acid and water,drying, vacuum treatment and sieving are mixed in a mixer with powdersof cesium chromate and/or cesium bichromate similarly subjected to theaforesaid two-step sieving in such a manner that these powders may notbe pulverized. The mixed powder is further dried and sieved to preventthe agglomeration of the powder. If required these mixing, drying, andsieving steps may be conducted in a vacuum vessel.

The mixed powder thus treated is satisfactory in respect of the particlesize, adsorbed water and gases and mutual contact, so that the resultantfluidity, reaction area and diffusion velocity thereof become constant,thus enabling cesium vapor to be uniformly deposited on a thin film ofsilver-bismuth or antimony constituting the metal base of the faceplate. Therefore the evaporator charged with such a mixed powderdisplays very excellent effects, for example, unfailingly depositing aprescribed amount of cesium vapor, and significantly improving theproperties of the face plate and the yield rate of photoelectric tubes.

There will now be described the method of depositing cesium vaporsaccording to an embodiment of the present invention.

2,500 g. of 99.9 percent pure silicon powder having a nearly uniformparticle shape was sieved with a 200 Tyler mesh screen. 500 g. of thepowder thus sieved was washed with a solution of 10 percent hydrochloricacid, and then completely stripped of the deposited hydrochloric acidwith distilled water. This portion of the power was dried 60 minutes ina thermostatically controlled oven maintained at a temperature of C.,and was introduced into a 200 ml. stainless steel pot together with 50stainless steel balls, and repetitively subjected to eight cycles of 30minutes pulverizing treatment by rotating the pot at a velocity of 100r.p.m. Between these pulverizing treatment was introduced a drying stepof 10 minutes in a thermostatically controlled oven maintained at 100 C.The silicon powder thus treated was measured to have an average particlesize of 60 microns, larger particles than 100 microns accounting for 15weight percent and those smaller than 1 micron, 20 weight percent. Thesilicon powder having such particle size distribution was introducedinto a suitable vessel placed in a vacuum region. When evacuation wascarried out and the vacuum region reached a vacuum of 3 l0"" torr,heating was applied for degassing 1 hour at 850 C. using an electricheater with the pressure within the aforesaid vessel maintained withinthe range of from 3X10 to 9X10 torr. Upon completion of heating, thepowder was allowed to cool 3 hours under vacuum. Next, while projectinglight beams from an infrared-ray lamp, the powder was allowed to passthrough a 150 Tyler mesh screen.

At the same time, cesium chromate powder was repeatedly subjected topulverizing and drying as in the case of silicon powder until the cesiumchromate powder had an average particle size of 6 microns and particleslarger than 25 microns accounted for 8 weight percent and those smallerthan 0.5 micron represented 0.4 weight percent. The powder was sievedunder the irradiation of an incandescent lamp in two steps first with a60 Tyler mesh screen and then with a Tyler mesh screen.

Two parts by weight of the silicon powder and 1 part by weight of cesiumchromate powder thus obtained were introduced into a V-type mixer andmixed at a velocity of 50 r.p.m. without substantially pulverizing thesetwo kinds of powders. The mixing operation was intermittently conductedover a total period of 3 hours by introducing between the operations adrying step of 30 minutes at 100 C. and a sieving step with a 60 Tylermesh screen under the irradiation of an infrared-ray lamp.

The above processes may be schematically represented as shown in FIG. 8.Namely, silicon powder 21 was processed in the order of sieving 22,washing with acid 23, washing with water 24, drying 25, several times ofpulverizing and drying 26, vacuum treatment 27 and sieving 28, whilecesium chromate 29 was processed in the order of several times ofpulverizing and drying 30, the first sieving by coarse screen 31 and thesecond sieving by fine screen 32. This was followed by mixing process33, drying 34, sieving 35 and mixing process 36. If required theprocesses of 33, 34, 35 and 36 may further be repeated to finally obtainthe mixture 37.

The evaporator of FIG. 1 filled with the mixed powder thus treated waselectrically heated to deposit cesium vapor on the face plate of aphotoelectric tube, the yield rate of good quality face plates being 98to 100 percent. Another experiment was carried out under the sameconditions as in the foregoing example, excepting that the cesiumchromate used in said example was replaced by cesium bichromate. In thiscase there were also obtained substantially the same results. By way ofcomparison, the deposition of cesium vapor was carried out by employinga mixture which was omitted of the intermediate steps of drying the rawpowders, heating the silicon powder under vacuum and sieving the cesiumchromate powder in two stages between the respective cycles of theaforementioned pulverizing operation. In this case, the yield rate ofgood quality face plates sharply declined to 40 to 95 percent.

While the invention has been described in connection with some preferredembodiments thereof, the invention is not limited thereto and includesany modifications and alternations which fall within the scope of theinvention as defined in the appended claims,

What is claimed is:

l. A method for preparing a source for generating cesium vapor fromwhich cesium is evaporated at a constant rate which comprises:

1. preparing a silicon powder having a particle size distribution suchthat the average particle size is between 30 and 80 microns and in whichparticles larger than 100 microns constitute not more than 30 weightpercent of the mixture and particles smaller than 1 micron constitutenot more than 30 weight percent of the mixture by repeatedly pulverizingand drying said silicon powder until a mixture with the stated particlesize distribution is obtained;

degassing and removing any adsorbed moisture from the resulting powderby subjecting the powder to a vacuum;

and then sieving said dried, degassed powder;

2. preparing a powder of at least one cesium compound selected from thegroup consisting of cesium chromate and cesium bichromate, said powderhaving a particle size distribution such that the average particle sizeranges between 3 and 10 microns and not more than 30 percent of themixture by weight consists of particles larger than 25 microns and notmore than 40 percent of the mixture by weight consists of particlessmaller than 0.5 micron, by repeatedly pulverizing and drying saidcesium compound powder until a mixture having the stated particle sizedistribution is obtained; 1

subjecting said powder to a first sieving operation through a relativelycoarse mesh screen to break up any large aggregates of powder in saidmixture;

and then subjecting said screened powder to a second sieving operationthrough a relatively fine screen;

3. mixing the silicon powder obtained in l) with the cesium compoundpowder obtained in (2) to form a mixture consisting of said siliconpowder and said cesium compound powder; and

4. drying and sieving the mixture so obtained to produce a mixture fromwhich cesium is evaporated at a uniform rate.

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2. preparing a powder of at least one cesium compound selected from thegroup consisting of cesium chromate and cesium bichromate, said powderhaving a particle size distribution such that the average particle sizeranges between 3 and 10 microns and not more than 30 percent of themixture by weight consists of particles larger than 25 microns and notmore than 40 percent of the mixture by weight consists of particlessmaller than 0.5 micron, by repeatedly pulverizing and drying saidcesium compound powder until a mixture having the stated particle sizedistribution is obtained; subjecting said powder to a first sievingoperation through a relatively coarse mesh screen to break up any largeaggregates of powder in said mixture; and then subjecting said screenedpowder to a second sieving operation through a relatively fine screen;3. mixing the silicon powder obtained in (1) with the cesium compoundpowder obtained in (2) to form a mixture consisting of said siliconpowder and said cesium compound powder; and
 4. drying and sieving themixture so obtained to produce a mixture from which cesium is evaporatedat a uniform rate.